The present invention relates to an image pickup apparatus such as a digital camera and a video camera, and more particularly to an image pickup apparatus which performs phase difference detection by using an image pickup element.
There is known a TTL phase difference detection method as an autofocus (AF) method used in an image pickup apparatus. In the TTL phase difference detection method, a part of a light flux from an image pickup optical system is divided into two light fluxes and a shift amount (phase difference) between two images formed by the two divided light fluxes is obtained. Based on the phase difference, a defocus amount of the image pickup optical system is calculated, and a driving amount of a focus lens necessary for obtaining an in-focus state by approaching the defocus amount to 0 is calculated.
In order to divide a light flux that has passed through an exit pupil of the image pickup optical system to obtain signals corresponding to the divided light fluxes, in many cases, an optical path dividing part such as a quick return mirror or a half mirror is provided in an optical path, and a focus detection optical system and an AF sensor are provided posterior to the optical path dividing part. Such a phase difference detection method which uses the AF sensor different from the image pickup element for obtaining a captured image will hereinafter be referred to as “sensor separate type phase difference detection method”.
The sensor separate type phase difference detection method enables direct calculation of the driving amount of the focus lens necessary for obtaining an in-focus state. Thus, a focusing operation can be performed within a short period of time. However, the light fluxes forming the two images on the AF sensor through the image pickup optical system and the focus detection optical system and a light flux forming an object image on the image pickup element through the image pickup optical system are different from each other, and hence a correction mechanism for correcting a difference therebetween is necessary, and in-focus accuracy is influenced by accuracy of the correction mechanism.
As another AF method, there is known a contrast detection method. In the contrast detection method, a high-frequency component is extracted from a video signal generated based on an output signal of the image pickup element, and the focus lens is driven in a direction of a level of the high-frequency component toward a peak. When the level of the high-frequency component eventually reaches a predetermined peak range, an in-focus state is determined.
In such a contrast detection method, AF is performed by using the video signal obtained based on the output signal of the image pickup element. Thus, higher in-focus accuracy can be obtained as compared with the sensor separate type phase difference detection method.
However, since the contrast detection method cannot directly calculate the driving amount of the focus lens necessary for obtaining an in-focus state, which is different from the sensor separate type phase difference detection method, the contrast detection method requires a long time to obtain the in-focus state.
Each of Japanese Patent Laid-Open Nos. 2003-295047 and 2000-292684 discloses an AF method in which advantages of such two different AF methods are combined. In the AF method disclosed in Japanese Patent Laid-Open Nos. 2003-295047 and 2000-292684, when an in-focus state is obtained by the contrast detection method, a defocus amount calculated by the sensor separate type phase difference detection method is stored as a correction value in a memory.
After such an adjusting operation, the defocus amount calculated by the sensor separate type phase difference detection method or the driving amount of the focus lens is corrected by using the correction value. This makes it possible to perform AF by the sensor separate type phase difference detection method more accurately.
However, the AF method disclosed in Japanese Patent Laid-Open Nos. 2003-295047 and 2000-292684 needs to perform, immediately after performing AF by the contrast detection method, AF by the sensor separate type phase difference detection method. Therefore, the AF method requires a long time until a series of these adjusting operations is completed.
Further, in order to obtain an in-focus state as accurately as possible by AF of the contrast detection method, it is necessary to prepare a specific object such as a dedicated chart and illumination for obtaining sufficient brightness of an object during the adjusting operations. As a result, a user needs to set an exclusive operation mode for performing the adjusting operations or prepare an exclusive environment, which necessitates time and labor.